The present invention relates to cellular communication systems and in particular to intercell handover procedures employed in such systems.
In wireless radio communication systems it is important to maintain a good quality radio link between the mobile station and the fixed station if satisfactory communication is to be achieved. If the system is a cellular system, the fixed station will form part of a fixed communications system having a number of radio terminations. One requirement for maintaining a good quality link is that the mobile station should establish a radio link with the closest radio termination whenever possible. By using the shortest possible radio link distance, the radio signal strength detected at the mobile station arisu due to transmissions at a given power from the appropriate radio termination will be maximised. The same will be true for the radio signal strength detected at the radio termination due to transmissions of the mobile station. Higher detected radio signal strengths contribute to establishing a good radio communications link.
As a mobile station is moved around the radio coverage domain of the cellular communication system, the mobile station may move from one position in which it is closest to a first radio termination of the system, to another position in which it is closest to a second radio termination of the system. Since the mobile station should communicate with the fixed communication system over the shortest possible radio link distance wherever possible, any mobile station which is supporting a communication (such as a voice call) that moves as described will initially support the communication on a radio link with the first radio termination and subsequently switch to supporting the communication on a radio link with the second radio termination. Such a switching operation is known as a handover operation.
In a cellular system, the radio coverage area provided by one radio termination is often known as a cell. Handover operations can be from one cell to another cell and such a handover operation is called an intercell handover. Normally handover operations between cells will be between adjacent cells. Another type of handover exists called intracell handover where a communication supported on a first radio link with a radio termination is switched to being supported on a second radio link but with the same radio termination. This type of handover allows one radio link to be replaced by another radio link without changing the radio termination with which the mobile station is communicating.
In some types of wireless radio communications system such as certain cellular telephone communication systems it is the responsibility of the mobile station to maintain a good radio link with the fixed station. Where the communication system is a cellular communication system this will involve determining which radio termination the link should be established with. An example of one such system is a DECT compliant telecommunications system. (DECT is an abbreviation for Digital Enhanced Cordless Telecommunications and DECT systems are described in the standard ETS 300 175 of which there are several parts. The standard is published by the European Telecommunications Standards Institute and is incorporated herein by reference.)
In a DECT compliant telecommunications system the mobile station is called the portable part (PP) and the fixed station is called the fixed part (FP). The fixed part includes at least one radio fixed part (RFP) which is a radio termination, or radio end point, the latter two terms being used synonymously.
Taking a DECT compliant system as an example of a system where the mobile terminal assumes responsibility for maintaining link quality, and controlling handover operations, the portable part may monitor the signal strength of the radio link in use that is supporting a particular communication, such as a voice call, to determine if a handover operation should be performed to support the communication on a new radio link established with a different radio termination. (The different radio termination should be closer to the mobile terminal if the signal strength is higher.) In the DECT example the handover would be to another RFP. Monitoring of the radio link in use is normally conducted on an ongoing basis and when a degradation in signal strength is detected, handover to the nearest adjacent RFP is attempted.
A problem with the above type of operation is that the portable terminal, in this case the PP, waits until the signal strength degrades before an intercell handover operation is attempted. By the time the handover operation is complete, the signal strength of the radio link connection may have reduced to such an extent that the quality of the radio link is also affected and there is perceptible degradation in the quality of the supported communication channel. Where the connection carries traffic in the form of a voice-call, the degradation may be apparent to the user. Where the connection is carrying data, this may result in a reduction of bandwidth.
It is an object of the present invention to reduce the probability of a noticeable degradation in communication channel quality occurring during handover operations.
In accordance with one aspect of the present invention there is provided apparatus for initiating a handover procedure in a cellular communication system including a portable terminal and a fixed terminal having more than one radio end point, the radio end points being geographically spaced apart and provided for establishing wireless radio communication with the portable terminal over a wireless radio link, said apparatus including:
means for monitoring the radio signal strength received at the portable terminal due to transmissions of the radio end points, and
initiating means for initiating a handover procedure to move a supported communication channel from being supported on a wireless link established with a first one radio end point to being supported on a wireless link established with a selected other one radio end point, the initiating means initiating the procedure to select the said other one radio end point immediately on determination by the monitoring means that the said other one radio end point provides a higher received signal strength than the currently selected radio end point.
Advantageously, because the said one other radio end point is selected as soon as it is determined that a higher radio signal strength is being received from it, the communication channel will be supported on a wireless link having the strongest signal strength for a greater proportion of time than would be the case for a communication system not benefiting from the present invention. The use of a wireless link having the strongest possible signal strength contributes towards providing a high quality communication channel.
In contrast with the operation of known communication systems, a communication system benefiting from the present invention will not wait for the radio signal strength of a selected wireless link to fall below a given threshold, before looking to use a wireless link having a higher signal strength. The present invention provides a mode of operation where handover attempts are made to pre-empt the onset of any degradation in the communication channel quality resulting from reduced radio signal strength of the radio link that is supporting the communication channel.
For such prior art systems it could occur to the skilled person that it may be possible to set the threshold level sufficiently high so that the system looks to use a new wireless link having a higher signal strength before the signal strength of the link in use falls enough to affect the quality of the communications link. However, it is a deterioration in signal strength which initiates the handover attempt and by the time a so initiated handover operation has been completed it is possible that a further deterioration has occurred which will cause a noticeable degradation in the communications channel quality, despite the fact the handover operation was initiated before the onset of such a degradation. In contrast, in the present invention handover operations are initiated on the basis that a radio link can be established with a higher signal strength than the radio link presently in use, not on the basis that the signal strength of the radio link presently in use is below a particular threshold.
In general the present invention will cause the portable terminal to maintain a wireless link with the closest radio end point and handover operations will normally be from one radio end point to a nearest adjacent radio end point.
The monitoring means can maintain information relating to the received radio signal strength in the form of values which may be stored in an array. In this case where the communications system employs predefined system communication channels, the values may be stored in positions in the array such that each position is assigned to a particular channel.
When the values are stored in an array having positions assigned to particular channels, there is no need to store channel identity information together with the radio signal strength information providing that the allocation of array positions to channels is known. The array is therefore an efficient way of storing information since it requires a comparatively small amount of storage capacity.
In accordance with another aspect of the present invention there is provided a method of initiating a handover procedure in a cellular communication system including a portable terminal and a fixed terminal having more than one radio end point, the radio end points being geographically spaced apart and provided for establishing wireless radio communication with the portable terminal over a wireless radio link, said method including the steps of monitoring the radio signal strength received at the portable terminal due to transmissions of the radio end points, and initiating a handover procedure to move a supported communication channel from being supported on a wireless link established with a first one radio end point to being supported on a wireless link established with a selected other one radio end point, the procedure being initiated to select the said other one radio end point immediately on determination that the said other one radio end point provides a higher received signal strength than the currently selected radio end point.
Other aspects and optional features of the present invention appear in the appended claims, to which reference should now be made and the disclosure of which is incorporated herein by reference.